The present invention relates to carburetors and in particular to carburetors for small gasoline engines, such as those used in the lawn and garden industry.
The necessity to meet emission levels required by government agencies such as CARB and the Environmental Protection Agency has resulted in the need to calibrate carburetors to operate under leaner conditions. Such leaner calibrations have caused most engines to start more poorly than the prior carburetors calibrated to produce a richer air-fuel mixture, and have been shown to cause false starts and surging during warm up from a cold start.
A temporary richer fuel delivery curve is required during these periods, but the simple fuel delivery system of current small engine carburetors do not compensate for such starting conditions. Furthermore, the primer systems of current small engine carburetors provide adequate fuel to start the engine, but the prime charge does not last long enough to carry the engine through warm up, even with repeated primes.
U.S. Pat. Nos. 2,744,736; 3,345,045; 5,058,544; 1,327,430; 1,206,221; and 1,562,806 disclose primer systems of small engine carburetors. For example, U.S. Pat. No. 5,058,544 discloses a floatless carburetor with an integral primer system which has a primer bulb with a primer chamber to introduce fuel directly into the carburetor throttle bore. This system may provide an adequate fuel charge to the engine for start up; however, the fuel charge from the primer chamber is sometimes not enough to last the engine through warm up, causing engine stalling, false starts and surging. Repeated fuel charges may be necessary in order to provide adequate fuel to last the engine through warm up.
U.S. Pat. No. 3,345,045 discloses a conventional carburetor with a priming system for an internal combustion engine. The carburetor has a float bowl with an air inlet above the fluid level. A flexible tube connects the interior of the float bowl with an air pumping device such as a flexible bellows. In order to provide a rich mixture needed for starting the engine, the bellows is manually depressed which increases pressure in the float bowl above atmospheric pressure causing a measured amount of fuel to flow from the float bowl upwardly through a fuel nozzle into a mixture passage. This system also may provide an adequate fuel charge to the engine for start up, but the provided rich mixture may not be enough to keep the engine running through the warm up period. Again, repeated fuel charges may be necessary.
Current small engine carburetors that have systems for priming through warm up are complex systems causing higher costs, having less durability, and possibly requiring additional activation of the primer. For example U.S. Pat. Nos. 4,836,157; 4,446,080; and 3,872,851 each disclose a carburetor with a priming system which includes electrical devices. U.S. Pat. No. 4,836,157 discloses a priming system which includes a temperature sensor connected to the engine and a pressure switch connected in the fuel line to provide signals to a priming control circuitry whenever fuel is pumped to the carburetor air intake. When signals exceed a selected threshold, an LED is driven by an oscillator to advise the operator to terminate the manual priming operation.
The inclusion of electronic devices to these priming systems results in undesirably high manufacturing costs and unnecessarily high maintenance costs. This is primarily due to the added time to manufacture the system and the costs of the electronic components. Additionally, with the implementation of electronic devices thereto, the durability of these small engines are limited by the constraints of the electronic devices. During normal operations of these systems, the user is required to be even more cognizant of the physical impacts endured by the engine due to the limiting factors of the electronic devices.
Current small engine carburetors that have systems for priming through warm up are dependent on parameters and/or conditions of the engine. For example, U.S. Pat. No. 3,780,996 discloses a self-priming chokeless carburetor having a throttle valve in the air-fuel mixture conduit supplemented by a priming well disposed serially between a fixed metering orifice communicating with a float bowl reservoir and an inlet to a main nozzle tube. However, the level of fuel in the priming well is dependent upon the level of fuel in the float bowl. Thus, the level in the priming well may or may not be adequate to last through engine warm up.
Current small engine carburetors that have systems for priming through warm up are complex systems manually operated. For example, U.S. Pat. No. 5,740,781 discloses a starting system constructed to utilize atmospheric air control feed back to a carburetor and includes a piston disposed with a chamber which accesses two atmospheric ports by manual selective movement of the piston. The system has a fuel prime arrangement which includes a primer pawl interconnected to the piston which depresses a prime bulb upon manual selective movement of the piston.
Thus it is desired to provide a simple primer system for small engine carburetors that provides an adequate amount of fuel to the engine for start up to last through warm up, but yet is low in cost, durable, and automated from engine start up through warm up.